strata is clear and decisive. In many cases, as at Walton, in Essex, and Fisherton, near Salisbury, the former underlie the latter, and must therefore be older.
In other cases they form part of the basin in which the Prehistoric deposits lie, as in the case of the gravel-beds of Windsor or of London, and must therefore, from their position, be of higher antiquity than the latter. Nearly every valley in Great Britain (the glacier-areas, to which I shall return presently, being excepted) contains beds of brick-earth or of gravel, which were formed, as Mr. Prestwich[1] has clearly shown, before the valleys were cut by the streams to their present depths; and the difference between the levels of these old river-terraces has been shown by that eminent observer to be a rough measure of their relative antiquity, the highest being the oldest. The Prehistoric deposits, on the other hand, occupy for the most part the bottom of the valleys, and are seldom raised much above the level of the present stream. There is also a marked difference between the two in the materials of which they are composed. The Prehistoric as well as the present alluvia are for the most part formed of clays, more or less stiff; and the gravels are composed of pebbles more or less evenly sorted; and both were formed under conditions of climate not very different from the present. The Pleistocene brick-earths, on the other hand, very seldom consist of stiff clays; and the gravels contain large and small pebbles and angular blocks confusedly mixed together, which indicate that the conditions under which they were formed were different from those which are now presented by the temperate region of Europe. But the difference offered by the fauna which they present is, perhaps, the most striking.
It will be unnecessary to give the river-deposits in Great Britain which have furnished the remains of the following animals, since they have already been published in the Quarterly Journal of the Geological Society, vol. xxv. pp. 192 et seqq.
It would, of course, be unreasonable to expect that the remains of all the animals inhabiting the country at the time would be present in one small river-deposit, and still more improbable that they would all be discovered in the small area which happens to be open for examination. By correlating, however, the animals from many localities, a fair estimate can be obtained of the whole fauna. The greater liability of one animal to drowning than another must be taken into consideration. For this reason the Otter, probably from its aquatic habits, is extremely rare, while the Squirrel, living in trees, would run little risk of a watery grave, and has only been found in one Pleistocene deposit in Great Britain. Altogether the following twenty-eight species of animals have left their remains to prove that they existed on the surface of the Pleistocene continent that was drained by the rivers in the deposits of which their remains have been found.
- ↑ Philosophical Transactions, vol. cliv.
